Andres Freund <andres@2ndquadrant.com> writes:
> On 2014-01-13 15:19:06 -0500, Tom Lane wrote:
>> The important point is not just that it not damage the page, but that
>> it not log it as invalid. I concur that the right fix requires a
>> new operating mode for XLogReadBufferExtended, perhaps RBM_NORMAL_ZERO_OK.
>> I think the spec for this should be that if the page doesn't exist or
>> contains zeroes, we return InvalidBuffer without logging the page number
>> as invalid. The doesn't-exist case is justified by the expectation that
>> there will be a later RBM_NORMAL call for a larger page number, which will
>> result in a suitable complaint if the page range isn't there.
> That's a sensible way to go, yes. But I wonder if we wouldn't end up
> with less complicated code if we added a variant of ReadBuffer that only
> returns a buffer from cache if already present in s_b.
That sounds rather invasive to me, and more to the point it requires
fooling with code that doesn't have anything to do with WAL replay.
> I started to prototype something like RBM_NORMAL_ZERO_OK shortly after
> Heikki's message and it seems to quickly turn a bit ugly because the
> surrounding code isn't really ready to deal with not returning a
> buffer. And for the purpose of that redo routine, that'd actually be
> better.
It looks easy enough to me to support the desired behavior in
XLogReadBufferExtended. I wasn't planning to support it in ReadBuffer
itself. (Probably the path of least resistance is to define it as meaning
the same thing as RBM_NORMAL, so far as bufmgr.c is concerned.)
regards, tom lane